<div id="app">
  <div class="box" :style="{width: `${R}px`,backgroundColor: `rgb(${R}, ${G}, ${B})`}"></div>
  <div>
    <input type="range" min="0" max="255" v-model="R"> R: {{R}} <br>
    <input type="range" min="0" max="255" v-model="G"> G: {{G}} <br>
    <input type="range" min="0" max="255" v-model="B"> B: {{B}} <br>
  </div>
</div>
<script src="https://unpkg.com/vue@next"></script>
<script>
  Vue.createApp({
    data() {
      return {
        R: 0,
        G: 0,
        B: 0
      }
    }
  }).mount('#app')
</script>

<style>
  .box {
    width: 100px;
    height: 100px;
  }
</style>